(1) Nucleotide analogs have been synthesized for use as structural probes of glutamine synthetase from Escherichia coli in order to obtain intra- and intersubunit distances in the dodecameric enzyme. Various analogs of ATP that are substituted at the 6- or 8-position of the adenine ring have been further modified with spectrophotometric and fluorometric probes or an electron dense Pt(II) marker for electron microscopy and for X-ray crystallographic studies. Active sites of this enzyme appear to be at the outer center edge of each subunit in the hexagonal rings; heterologously bonded intra-ring subunits have active sites about 57 angstroms apart (which is almost the maximum distance possible) and isologously bonded subunits from opposing rings have sites located at the exterior about 75 angstroms apart. (2) Glutamine synthtase active sites also have been covalently labeled and lysyl residue 47 has been found to be near the subunit ATP binding site of the E. coli. enzyme. (3) Active-site ligand and metal ion interactions with mammalian glutamine synthetase are being studied to define structural and catalytic roles of divalent cations. (4) Thermodynamic parameters for the sequential binding of active-site ligands to E. coli glutamine synthetase have been obtained by calorimetry, equiligrium binding, and fluorescence titrations. (5) Studies on the mercurial-promoted release of Zn2+ from E. coli asparate transcarbamoylase (ATCase) have been completed. Studies on the rebinding of Zn2+ ions to isolated regulatory subunits of this enzyme are in progress. The results relate to changes in catalytic and regulatory chain interactions in ATAcase molecules responsible for the allosteric properties of this enzyme and to mechanisms of dissociation and assembly of this enzyme.